events: Introduce EventRewriter.

ui/events/event_rewriter.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UI_EVENTS_EVENT_REWRITER_H_
+#define UI_EVENTS_EVENT_REWRITER_H_
+
+#include "base/memory/scoped_ptr.h"
+#include "ui/events/events_export.h"
+
+namespace ui {
+
+class Event;
+
+// Return status of EventRewriter operations; see that class below.
+enum EventRewriteStatus {
+  // Nothing was done; no rewritten event returned. Pass the original
+  // event to later rewriters, or send it to the EventProcessor if this
+  // was the final rewriter.
+  EVENT_REWRITE_CONTINUE,
+
+  // The event has been rewritten. Send the rewritten event to the
+  // EventProcessor instead of the original event (without sending
+  // either to any later rewriters).
+  EVENT_REWRITE_REWRITTEN,
+
+  // The event should be discarded, neither passing it to any later
+  // rewriters nor sending it to the EventProcessor.
+  EVENT_REWRITE_DISCARD,
+
+  // The event has been rewritten. As for EVENT_REWRITE_REWRITTEN,
+  // send the rewritten event to the EventProcessor instead of the
+  // original event (without sending either to any later rewriters).
+  // In addition the rewriter has one or more additional new events
+  // to be retrieved using |NextDispatchEvent()| and sent to the
+  // EventProcessor.
+  EVENT_REWRITE_DISPATCH_ANOTHER,
+};
+
+// EventRewriter provides a mechanism for Events to be rewritten
+// before being dispatched from EventSource to EventProcessor.
+class EVENTS_EXPORT EventRewriter {
+ public:
+  virtual ~EventRewriter() {}
+
+  // Potentially rewrites (replaces) an event, or requests it be discarded.
+  // or discards an event. The arguments must not alias. If the rewriter
+  // wants to rewrite an event, and dispatch another event once the rewritten
+  // event is dispatched, it should return EVENT_REWRITE_DISPATCH_ANOTHER,
+  // and return the next event to dispatch from |NextDispatchEvent()|.
+  virtual EventRewriteStatus RewriteEvent(
+      const Event& event, scoped_ptr<Event>& rewritten_event) = 0;

[sadrul, 2014/03/26 17:26:27]

chromium doesn't allow non-const refs. So use a pointer for |rewritten_event|
instead.

You can remove 'arguments must not alias'. scoped_ptr<> is sufficient to convey
that the EventRewriter takes ownership of the event, and the caller should not
try to free it.

+
+  // Supplies an additional event to be dispatched. The arguments must not
+  // alias. It is only valid to call this after the immediately previous

[sadrul, 2014/03/26 17:26:27]

ditto

+  // call to |RewriteEvent()| or |NextDispatchEvent()| has returned
+  // EVENT_REWRITE_DISPATCH_ANOTHER. Should only return either
+  // EVENT_REWRITE_REWRITTEN or EVENT_REWRITE_DISPATCH_ANOTHER; otherwise
+  // the previous call should not have returned EVENT_REWRITE_DISPATCH_ANOTHER.
+  virtual EventRewriteStatus NextDispatchEvent(
+      const Event& last_event, scoped_ptr<Event>& new_event) = 0;
+};
+
+}  // namespace ui
+
+#endif  // UI_EVENTS_EVENT_REWRITER_H_